Camera Flicker / Optical ENF
Definition
The periodic brightness variation captured in video recorded under mains-powered fluorescent or LED lighting. The lighting modulates at 100 Hz or 120 Hz (twice the mains frequency), leaving an ENF signal encoded in the pixel intensity of video frames rather than in the audio channel.
- Frequency doubling
- Lighting modulates at 100 Hz or 120 Hz, twice mains frequency
- Encoded in
- Pixel intensity of video frames, not the audio channel
- Light source
- Mains-powered fluorescent or LED fixtures
- Use
- Timestamp verification of video via ENF database matching
Common questions
How does optical ENF differ from the standard audio ENF method?+
Standard ENF analysis extracts the mains hum from an audio track. Optical ENF instead reads brightness fluctuations from the light source captured on the video sensor, so it can validate footage that has no usable audio at all, such as silent CCTV.
What footage is unsuitable for optical ENF extraction?+
Video shot outdoors in daylight, under battery-powered or DC lighting, or at a frame rate too low to resolve the 100 or 120 Hz flicker will not yield a usable signal. The camera's rolling shutter readout rate also affects whether the flicker is captured cleanly.
Why does frame rate matter for detecting this signal?+
Recovering a periodic signal above roughly 50 to 60 Hz from a standard video frame rate requires exploiting how a rolling-shutter sensor scans lines sequentially, not the full-frame rate itself, since sampling the flicker frame by frame alone would violate the Nyquist limit.
Related terms
- Cross-Correlation
- A statistical measure of the similarity between two time series as a function of a time lag. In ENF matching, the extracted...
- Electric Network Frequency (ENF)
- The instantaneous frequency of the alternating-current mains supply, nominally 50 Hz in most of the world and 60 Hz in North America....
- ENF Reference Database
- A continuously maintained archive of grid frequency measurements recorded at high temporal resolution, used as the ground truth against which a recording's...
- ENF Splice Detection
- Using breaks or discontinuities in the ENF time series to identify edits, cuts, or inserted segments in a recording. A genuine unedited...
- Geographic Grid Disambiguation
- The process of determining which national or regional grid a recording's ENF trace came from, since the 50 Hz grids of continental...